Microbially induced calcium carbonate precipitation (MICP) can significantly improve the engineering mechanical properties of granite residual soil, but its solidification effect is greatly affected by the clay content. MICP curing test was carried out by adding different clay particles in granite residual soil. The calcium carbonate content of solidified soil samples was measured, and the unconfined compressive strength test was carried out to study the effect of different clay content on the curing effect. On this basis, nuclear magnetic resonance (NMR) test was carried out to study the curing mechanism of clay content on granite residual soil from the microscopic point of view. The results show that the strength of microbial solidified soil samples mainly includes the strength of soil skeleton and the strength of calcium carbonate cementation. The former is affected by the porosity of soil itself, and the latter affects the strength of solidified soil samples through the deposition of calcium carbonate crystals. With the increase of clay content, the porosity in the test soil sample is reduced, and the deposition of calcium carbonate is also reduced, and the strength after increase significantly. Adding appropriate amount of red clay (optimal clay content is 60%) can obtain higher curing strength without changing other properties of soil samples.